There are some good steam railways in the area, which are quite good even if it's wet (if you are actually on the train you are dry!). There's the Vale of Rheidol railway which runs from Aberystwyth to Devil's Bridge. The Corris railway (near Machynlleth) which is small but interesting, and a bit further away the Tal-y-llyn railway which is excellent.
There is also a butterfly centre quite near Aberystwyth which is inside a large polytunnel type thing. Oh, and there is Reptile World at New Quay (south of Aberystwyth) which has reptiles to look at and is indoors.
Thanks for the replies. Mejon That farm park looks good and thanks for the link to the soft play. I think the caravan park in Clarach is actually where my in-laws will be staying.
Shockers I really hope for nice sunny weather too (I say as it snows outside just over a month before we are going). I have been to Borth with PILs before, as it is literally the only place they go on holiday, and they are happy to just wander round Aberystwyth in the rain or stay in, which DD won't be so happy with.
Responding a little late sorry. I live near Aber so trying to think of things you could do. There are a couple of soft-play places nearby. One - Play Planet is in Llandre which is next to Borth. There's also a smaller one in Clarach in the centre attached to the caravan park. There's also a swimming pool there. Plascrug Leisure Centre in Aberystwyth itself has a small soft-play area and a pool. There's Fantasy Farm Park in Llanrhystud which has an indoor play area and some farm animals indoors. This Silver Mountain opened last year - I'm not sure how much of it is indoors though.
We are going to Borth near aberystwyth at the beginning of May. Based on the weather so far this year I expect it to be cold and wet. It will be DH and I and DD aged 2. In-laws (5 adults, a 2 yr old and a 9 month old) will be staying nearby. Is there anything at all to do in bad weather in the area? Our rented cottage is on the seafront, and I am happy to spend time on the beach even in the cold and wet if necessary, but not all the time. MIL says we will be able to use the swimming pool at the caravan site where they are staying, but DH won't swim and I'd like something all 3 of us can do. Are there indoor attractions that would interest a 2yr old? Is there a soft play place in case we get desperate?